It will be recalled that transmissions of information in an-ATM network are accomplished by means of cells composed of a 5-byte header and a 48-byte data block. The format of such a cell is represented in FIG. 1. Certain specific cells, which are recognizable from specific values of certain fields of the header, are reserved for maintenance operations and are designated, in accordance with the ATM standard, as F4 or F5 maintenance cells. The content of these cells is developed by the manufacturers particularly in order to supply redundant information which pertains to a set of user cells that are intended to be sent. Thus, a transmission of information occurs in the form of a block which comprises a predetermined number of user cells, and these cells are transmitted in succession, followed by a maintenance cell associated with the block.
In addition to t]he maintenance cells, the ATM standard makes it possible to introduce cells of the redundant data into the interior itself, which allows the detection of errors in transmission. This will be the case when, for example, the cells are assigned to the transmission of communications protocols for Swapping with local networks.
The ATM standard also allows time-division multiplexing at several levels while organizing the flow of cells into virtual paths (VP) and virtual channels (VC). Thus, each ATM cell can be assigned to one virtual path VP among 4,096 and to one virtual channel VC among 65,536. The virtual channel and path numbers of the cell are contained respectively in the 12- and 16-bit fields of the header.
In accordance with these two levels of multiplexing, the standard provides for specific maintenance cells F4 and F5 that are respectively assigned to the virtual paths VP and the virtual channels VC. These maintenance cells make it possible to perform verifications of the quality of the transmission either at the level of the virtual paths, or at that of the virtual channels. For that purpose, they contain check information such as parity bits which are calculated from the data in the user cells of the associated block.